appendix

• 주소 한 개가 가리키는 단위: 1 byte
• 주소 비트 수 → 주소 공간: 비트 수가 N이면 2^N개의 주소 표현
• 전체 메모리 용량: 각 주소가 1바이트씩 할당되므로, 총 2^N byte
• 16 bit → 64KB
• 32 bit → 4GB
• 주소 크기 = 프로그램 크기

Operating System

• A program that acts as an intermediary between a user of a computer and the computer hardware
  

• Resource manager and allocator
  • Efficient and Fair
  • e.g) 10 → 5 5 , 7 3 ; 5 5 faster ⇒ Fairness
• Control program
  • Controls execution of user programs
  • Prevents errors and improper use
• Many Diffent OSes
  

Two Common OS Families

Nowdays that’s basically everything

POSIX

Portable Operating System InterFace for Unix

• Anything Unix-ish
• e.g) Linex, BSDs, Mac, Android, iOS. QNX

Windows

• Stuff shipped by MicroSoft

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Basic Computer Architecture

• IBM PC Architecture x86
  • Until recently, most computers could still run MS-DOS based programs
  • Alternatives: Amiga, Macintosh, PowerPC
• Architecture determines many properties of the OS
  • load and take control
  • interacts with devices
  • manage CPU and memory

Some history

• 1981: IBM releases a PC to compete with Apple
  • Basic Input/Output System (BIOS) for low-level control
  • Three high-level OSes, including MS-DOS
  • Developers were asked to write software for DOS or BIOS,
    not bare-metal hardware
• 1982: Compaq releases IBM-compatible PCs ⇒ IBM Loses Control
  • intel 808x CPU, Different hardware implementations
  • Reverse enginerred BIOS, Customized version of MS-DOS
• 1985: IBM clones dominated computer sales
  • Same underlying CPUs and hardware chips
  • Close to 100% BIOS compatibility & MS-DOS was ubiquitous
  • Thus, IBM PC hardware became the de-facto standard
• 1986: Compaq introduces 80386-based PC (x86)
• 1990’s: Industry is dominated by “WinTel”
  • Intel x86 CPU architectures

Computer Component

CPU Central Processing Unit

• Physical standard < Intruction Set Architecture
  • IBM PCs are Intel 80836 compatible
  • Intel, AMD, VIA
• Today’s dominant ISA: x86-64, developed by AMD

RAM Random Access Memory

• ~ 1993: DRAM (Dynamic RAM)
  • Dynamic: refresh
• 1993 ~: SDRAM (Synchronous DRAM)
• Current standard: Double data rate SDRAM (DDR SDRAM)
  

North Bridge

• Coordinates acess to main memory
• CPU - Memory
• CPU access to I/O degrades performance

I/O device

ISA → PCI → PCI Express(serial, fast)

South Bridge

• Facilitates I/O between deveces, the CPU, and main memory
• CPU - I/O

Storage connectors

• Also controlled by the South Bridge
• Old standard: Parallel ATA
• Other standards
  • Small Computer System Interface
  • Serial ATA
    • SATA is faster than P-ATA

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Architecture Review

CPU

Now Storage Hierarchy

• Locality
  • OS manage data area, programmer manage GPU

Typical Memory Layout

• 16-bit CPU can address 64KB of memory
• 1 byte is stored at each address
• Not all memory is free

Communicating between CPU and Devices

1. Polling I/O - Synchronous

• I/O-only memory space shard by the CPU and a device
• CPU and device communicate by reading or wiriting to the virtual memory space
  • Each device is assigned some portion of the address space
• Polling forces the CPU to wait until the device is ready
  • continuous device check → synchronous
• Problems
  • CPU must mediate all transfers
    • [Case] Transfer data from disk to memory
    • CPU must copy value from the I/O port to memory → slow
  • All I/O must be synchronous
    • [Case] Disk wants to send data
    • but the CPU isn’t reading the I/O port
    • CPU가 지속적으로 입출력의 상태를 읽고 있어 다른 작업 수행 불가

2. DMA: Direct Memory Access

• Regions of RAM that are shared by the CPU and a device
• Enables devices to transfer blocks of data directly to memory
  • Interrupt generated when transfer is completed
• Much faster than the alternative method
  • Interrupt generated after each byte is transferred
    

3. Interrupts - Asynchronous

• Signal from a device to the CPU
• Interrupts causes the OS to switch to handler code
  • Each interrupts is assigned a number
    • 복잡한 프로그램은 번호 지정
  • Number acts as an index into the Interrupt Vector Table
• Interrupt causes a context switch
  - State of the CPU must be saved before the switch
  - and restored after the handler completes
  - Context means the Current task status is register value
  

Interrupt Timeline

• Interrupt enables the CPU and devices to work in parallel
  

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Booting Up

1. Starting the BIOS

Basic Input/Output System

• Code from the BIOS chip gets copied to RAM at a low address (e.g. 0xFF - 16 bits)
• jmp 0xFF written to RAM at 0xFFFF0
• x86 CPU always start with 0xFFFF0 in the EIP register

RAM 0xFF에 BIOS가 code copy, CPU가 시작하는 위치인 0xFFFF0에는 0xFF로 jmp하라는 명령어 존재

Essential goals of the BIOS

• Check hardware to make sure its functional
• Scan storage media for a Master Boot Record
  • Load the boot record into RAM
  • Tells the CPU to execute the loaded code

2. Load Settings from CMOS

3. Initialize devices

Scan and initializes hardware

• CPU and memory
• Keyboard and mouse

Builds the Interrupt Vector Table

4. Runs POST test(Power-On Self Test)

• 하드웨어가 잘 작동하는지 확인

5. initiate the bootstrap sequence

Bootstraping

• BIOS indenifies all poentially botable devices
• MBR has code(bootloader) that can load the actual OS
  • 요즘은 bootloader를 올리는 코드인 경우가 많음 - chain loading

Master Boot Record

• 512 byte file written to sector 1
• 446 bytes of executable code
• Entries for 4 partitions
• The magic number 55AA servers to identify the MBR
  

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Applications

System Call Interface

Interface between OS and Applications

System call table

• Layer of indirection to abstract the OS APIs
• Each OS API is given a specific index in the table
  • OS가 지원하는 함수 API로 table에서찾아서 jmp → IVT와 유사

Traps: Software Interrupts

Software can also generate interrupts

• 1 is system call number
• CPU calls 80 interrupt
• On x86, system calls are intiated via an interrupt
  • On x86, int 0x80 is the system call interrupt
  • EAX holds the table index of the desired API
• sys_read와 같은 system call은 직접 ssd에 접근하지 않고 커널로 제어 넘김
  • trap(interrupt)을 발생시켜 커널로 제어를 넘김

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Kernel

kernel

• The one program running at all times on the computer
  • Bootloader는 kernel을 memory에 road하는 역할

kernel features

Device Management

• Required: CPU and Memory
• Optional: disks, keyboards, mice, video, etc.

Loading and executing programs

System calls and APIs

Protection and Security

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Architecting Kernels

Monolithic kernels

• All functionality is compiled together
• All code runs in privileged kernel-space
• Pros
  • Sing code base
  • Robust APIs - System call이 정해져 있으면 일관성, 통합
  • Fast performance
• Cons
  • Large code base → Hard to manage
  • Bugs crash the entire kernel
    

Micro kernels

• Only essential functionality is compiled into the kernel
• All other functionally runs in unprivileged user space
• Pros
  • Small code base → Easy to manage, Context switching overhead 큼
  • Service may crash, but the system will remain stable
• Cons
  - Sloe performance - Many context switches
  - No stable APIs
  

Hybrid kernel

• Most functionality is compiled into the kernel
• Some functions are loaded dynamically